Mechanisms of formation of polyphosphates MeIII(PO3)3, where MIII=La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc, Fe, Ga, Al and Cr has been simulated by thermal analysis technique.
MeIII oxides and ammonium dibasic phosphate (NH4)2HPO4 were used as starting materials. For MIII=La-Lu, Y and Fe three main stages were observed: 1. elimination of water and ammonia leading to the formation of ammonium
tripolyphosphate (NH4)5P3O10; 2. reaction of the latter with Me2IIIO3 and formation of acidic polyphosphates MeIIIH2P3O10 or their isomers MeIII(PO3)3·H2O; 3. final loss of water and formation of MeIII(PO3)3. For MeIII=Sc and Ga the second stage is prolonged and the polyphosphates form at higher temperatures. Aluminum and chromium polyphosphates
are unstable. It is suggested that thermal behavior of the compounds is determined by MeIII ionic radii.